MyE28.com

I'm doing a rather...unconventional, experimental, insane, ill-advised, probably stupid build. It's not being done on a pristine car. It involves, among other things, a full set of 17x12 style 5s, 315 tires, a turbo, and really wide fender flares. It's my dream and kind of an extension of who I am, and is not the whim I cooked up yesterday, it's an idea I have been chewing on for the last 2 years.

The only comparison I can make it to is this drift car: http://www.mye28.com/viewtopic.php?t=126101 I'm in contact with the guy that built it. I know there's some pretty drastic, irreversible modifications I'm going to have to make.

"Take this crap to Stanceworks." Yeah, I know, if I took this crap to Stanceworks, I would get a dozen spamtard replies along the lines of "thank you for sharing this very useful information fortnitemobilegame.com", lawyer services in Houston and NewHotMobileGameApk2019. I literally complained once after a spambot bumped a 10 year old thread, and another spambot COPIED AND PASTED what I said.

So anyway, 17x12 wheels will increase the scrub radius immensely. I have no idea what kind of scrub radius e28s have to begin with. The drift car with the flipped wheels would effectively have had 4"/100mm higher scrub radius than stock 17x8 style 5s; if I run 315 tires all the way around with 17x12s, I'll have a scrub radius 2"/50mm higher than stock.

Question: what is the scrub radius on an e28, how does it compare to other cars, and what effect is it going to have given that our cars don't have typical sports car suspension? Thanks in advance. *puts flamesuit on*

I was curious so did some reading here: https://www.moderntiredealer.com/articl ... et-matters

The one takeaway for your situation I had was the following:

On a rear-drive car, acceleration and braking forces both have the same effect on the front wheels. If scrub radius is positive (contact patch arc outboard of the pivot point), the wheels will try to toe out.

No ideal for control when accelerating or looking to stop.

This sounds complicated, but upon some initial research, very interesting.

I believe this information is not widely known or documented for e28s, but the necessary information to calculate might be obtainable from various sources and plugged into a calculator I found here: https://www.racingaspirations.com/apps/ ... alculator/. Some manual measurements of specific parts might need to be made...

That calculator is pretty cool. Hoping I can find some specs on the stock scrub radius, I've scoured the internet best I can and came up with nothing.

Beemernut wrote:That calculator is pretty cool. Hoping I can find some specs on the stock scrub radius, I've scoured the internet best I can and came up with nothing.

You're not going to find specs without measuring some things yourself and calculating...doesn't look like people have looked into this for e28s before, because honestly, they're not great racecars.

Just had a eureka moment. I bet I could draw a straight line across my tire in tire chalk, rotate the wheel so the line faces down, steer the wheel back and forth a half dozen times, and examine where the pivot point appears. I think that would get me down to roughly quarter inch accuracy.

You may have to because changing ride height, wheel offset and other components will change these dimensions.

I'm just curious where you were able to find 17x12 style 5 wheels.

A few years ago, someone here on the board who lives in Europe built a 'csl batmobile' racecar version of an e28 with super wide wheels and flares, but I can't remember who it was...as I recall it worked pretty well on the track. Maybe someone else can provide a link, or you could do some searching to find the discussion.

Well, with the aforementioned method, I found that the swivel mark appears to be roughly 10mm towards the inside of the tire - 10mm positive scrub radius with stock Style 5's, no spacers. With a little shower math I have determined that style-5-based 17x12 wheels should add between 50 and 55 millimeters to the mix, placing it at around 60mm, which is the stock scrub for a Corvair, which is a good thing, right? Right? The grumpy old guy that comes into the shop every two weeks says Corvairs were the best handling cars ever back in their day, right?

stuartinmn wrote:I'm just curious where you were able to find 17x12 style 5 wheels.

Behold https://www.youtube.com/watch?v=h1W3EMmEuQc
Now I need just 2 more style 5 wheels.

May sound stupid, but that information isn't in owners manual? They had some odd facts in there when I flipped through it last...


I hear scrub radius all the time when people talk about spacers:

"5mm I wouldn't worry about, but trying to increase the track width by 20+mm will significantly change your scrub radius and make the car a real handful on turn-in at the track."

"To understand scrub radius you irst have to understand SAI -steering axis inclination. The steering axis(SA) is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. The angle between the SA and vertical is SAI, add static camber to that and you have the included angle.

The scrub radius(SR) is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.

older 911s have a small (+) SR, from 964 up they have a small(-) SR

the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability.

The bigger the scrub radius the more kickback there is at the steering wheel. If you change to wheels w/ larger o/s you will feel the increased kickback at the steering wheel


If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly(as used on 911 front) typically performs well with a lot of SAI and caster(sound familiar?), a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

abs equipped cars want a little (-) SR to allow for uneven braking forces between the left and right front wheels"

https://rennlist.com/forums/racing-and- ... dling.html




Supposedly "r" is scrub radius

So some threads about that, might help answer some questions. Drift people use spacers all the time to clear shit, so their might be some decently documented things in that part of interweb as well.

Consensus on most forums seems to indicate that the SR shouldn't be over 20-30mm, so I'm definitely going to be over the hairy edge of insanity with this build. I'll try to report back with the results when I get it done. From all the research I've done, I think I can gather that I'll have extremely heavy steering, a lot of unnecessary feedback, potential tramlining issues and more. The only hope that I have is that the CSL widebody drift rat build survived with twice the scrub radius. And I think the wide tires should make up a little in stability.

Extra negative camber should work in your favour. Static tire contact patch is smaller and SR decreases. Curious to find out how it all works out for you.

71717FF7-376A-4353-9AA1-0947392E76B1 by Tiit Saul, on Flickr


503AB92B-B24F-4FB5-ADF3-183AC55519D8 by Tiit Saul, on Flickr

Ok, I have a stupid question. If I apply enough camber, say, 2-4 degrees to negate these scrub radius effects and handle better, will it result in poor tread wear on the inside and make me go through tires twice as fast? My 318ti has visible camber, probably 3 degrees with the camber plates maxed out in front (and I hate how it looks with 205/55r16 Bridgestone Ecopias), but it handles nicely, and I haven't put enough mileage on it to see bad tire wear.

Adding negative camber should make the steering lighter. Negative camber actually make tires wear more evenly, given you drive it hard in corners. If you don’t drive it hard, then 315 tire is pretty god damn pointless.? I don’t care about tire wear for a long time. I drive until tires are worn and then I’ll get them replaced.